Code division multiple access mobile communication system

ABSTRACT

A code division multiple access (CDMA) mobile communication system with improvements to permit stable reception with a minimum of bit error. The system comprises a voltage-controlled oscillator for supplying a carrier to a radio frequency quadrature demodulator, and a frequency controller for detecting a frequency error from a phase correction signal of the first step to generate a control signal that controls the oscillator. The frequency controller includes an extracting circuit and an integrating circuit. The extracting circuit extracts a phase change based on the frequency error derived from the phase correction signal of the first step and from a signal preceding that signal by a predetermined delay time. The integrating circuit integrates the phase change and outputs the integrated result as the control signal. The predetermined delay time is preferably set within a range not exceeding the delay time needed for averaging by an averaging circuit that receives the phase correction signal of the first step and outputs a phase correction signal. The carrier to a radio frequency quadrature modulator is preferably supplied from the voltvage-controlled oscillator.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a mobile communication systemoperating on what is known as the code division multiple access (CDMA)system.

[0002] The CDMA system involves multiplexing a plurality ofcommunication channels using spread spectrum codes, each channel beingassigned a different spread spectrum code. A given signal to betransmitted is multiplied (i.e., spread) by the spread code assigned tothe own channel, and is multiplexed with other similarly spread signalson different channels before being transmitted. At a receiver, themultiplexed signals are multiplied (i.e., despread) by the same spreadcode so that only the target signal will be extracted correlated on theown channel. The signals on the other channels are perceived merely asnoise because these signals with their different spread codes remainuncorrelated. The level of the noise may be sufficiently lowered so asnot to disturb the signal reception. The CDMA system is attractingattention as a system fit for drastically improving the efficiency offrequency utilization and has been commercialized in some areas.

[0003] Where CDMA communication is implemented using spread codes, somekind of signal modulation (e.g., quadrature phase shift keying or QPSK)precedes the spreading of the signal for transmission. At a receivingpoint, the despreading of the signal is followed by demodulation.Despreading and demodulation both represent the detection processwhereby the transmitted signal is reconstructed. Commonly used detectionmethods include a coherent detection method based on the PLL (phaselocked loop) circuit and a differential detection method. There alsoexists a recently proposed coherent detection method that utilizes pilotsignals.

[0004] Where the CDMA system is applied to a mobile communication systemadopting the conventional coherent detection method, the bit error rateof data in a mobile station deteriorates if a fading occurs while thestation is moving. In a CDMA mobile communication system utilizing thedifferential detection method, the bit error rate of data in a mobilestation can worsen due to the noise on the air transmission channel evenif the station is stationary. The pilot signal-based coherent detectionmethod has been proposed for a system to minimize the deterioration ofthe bit error rate whether the mobile station is in motion or at rest.The method was discussed at the Autumn 1994 Symposium of the Instituteof Electronics, Information and Communication Engineers of Japan asdisclosed in the IEICE collection of papers B-5 on radio communicationsystems A and B, p. 306, “Coherent detection for CDMA MobileCommunication Systems” by Yasuo Ohgoshi et al.

[0005] Described below is a conventional mobile communication systemthat uses pilot signals with reference to the above-cited papersupplemented by some details. The description will first center on thedown link of the system (i.e., a link from the base station to a mobilestation). FIG. 13 shows a modulation circuit 51 of a base station 1 thattransmits data and a first half 52 of the detection circuit of a mobilestation 2. The base station 1 actually transmits signals to a pluralityof mobile stations 2, and FIG. 13 shows one station as therepresentative example.

[0006] In the modulation circuit 51 (left-hand half of FIG. 13), datafirst undergoes QPSK modulation, not shown, to divide into an in-phasesignal I and a quadrature signal Q. The signals I and Q are spread(i.e., multiplied) respectively by spread code signals PN_(—ID) andPN_(—QD). The two spread code signals are supplied from a spread codegenerator 91. The rates of the spread code signals PN_(—ID) and PN_(—QD)(called the chip rates) are used to multiply by k (k: spreading ratio)the pre-spread rates (called the symbol rates) of the signals I and Q sothat the latter will attain the chip rates. The signals thus spread passthrough a radio frequency quadrature modulator 54 to become mutuallyperpendicular signals that are transmitted on a radio frequency bandfrom an antenna. A temperature compensated crystal oscillator 61 isprovided to furnish the modulator 54 with a carrier C_(B.)

[0007] The pilot signals will now be described. The transmission circuitis substantially the same as the left-hand half of FIG. 13 and isomitted. An in-phase signal I_(p), and a quadrature signal Q_(p) of thepilot signals are spread respectively by spread code signals PN_(—IP)and PN_(—QP). Both spread code signals have the same chip rate as in thecase of data. The pilot signals thus spread are subject to radiofrequency quadrature modulation by the same carrier C_(B) as with data,turning into mutually perpendicular signals transmitted on the sameradio frequency band as with data. The pilot signals serve as referencesignals for demodulation and are common to all channels utilized.

[0008] In the first half 52 (right-hand half of FIG. 13) of thedetection circuit of the mobile station 2, the received signals from theantenna (data and the pilot signals) pass through a radio frequencyquadrature demodulator 57 to reach a low-pass filter 56. The low-passfilter 56 removes the radio frequency components from the signals toyield signals S_(I) and S_(Q). A crystal oscillator 60 supplies thedemodulator 57 with a carrier C_(m). The signals S_(I) and S_(Q) arecomposed of the spread signals I and Q (those destined to the ownchannel as well as to other channels) and of the spread pilot signalsI_(p), and Q_(p). As such, the signals S_(I) and S_(Q) include a phaseerror caused by fading and a frequency error attributable to theprecision of the oscillator 60.

[0009] The errors included in the signals S_(I) and S_(Q) produce aphase difference therein. When the mutually perpendicular pilot signalsare plotted in orthogonal coordinates, the received pilot signals arerotated exactly by the phase shift, as shown in FIG. 14. If the phaseshift is represented by ø and the orthogonal coordinates afterquadrature demodulation are designated by X₁ and Y₁, then the coordinateaxes X and Y of the received signals are rotated by ø displacing thepilot signals. Consequently, the undisplaced signals i and q that shouldhave resulted with no phase shift become i₁ and q₁ respectively. Suchchanges are caused by the mixing of one of the two mutuallyperpendicular signals into the other signal. The phenomenon is expressedby the following formulas:

i_(i)=icosø−qsinø  

q₁=qcosø+isinø  

[0010] The pilot signals are signals that stay constant following thedespreading. Generally, i=1 and q=1. The signal changes into i₁ and q₁,permit acquisition of a signal CS with the value cost and a signal SNwith the value sinø. With the two signals known, it is possible tocorrect the phase rotation of the data. Since the data includes the samephase shift, the despread data signals are inversely rotated by ø usingthe signals CS and SN whereby the initial signals I and Q are correctlyreconstructed. Thus the signals CS and SN serve as phase correctionsignals.

[0011] The signals S_(I) and S_(Q) output by the first half 52 of thedetection circuit are subject to despreading and phase correction by thesecond half of the detection circuit shown in FIG. 15. A pilot signaldespreading unit 21 in the upper left portion of FIG. 15 despreads thesignals S_(I) and S_(Q) by use of the spread code signals PN_(—IP) andPN_(QP) from a spread code generator 25, whereby the pilot signals areextracted. The extracted pilot signals are then added and subtractedmutually, becoming a signal CS_(c) with a chip rate of cost and a signalSN_(c) with a chip rate of sinø. The two signals are converted to thesymbol rates by an accumulator 41 and thereby turn into phase correctionsignals C_(S). and SN_(S) of the preliminary stage. The phase correctionsignals are averaged by an averaging circuit 43 for noise reduction. Theaveraging provides the phase correction signals CS and SN of the finalstage.

[0012]FIG. 16 shows a typical circuit constitution of the averagingcircuit 43. Reference numerals 430 through 433 are delay gates (Ds) fordelaying a signal by a one-symbol period each. In this example, threeconsecutive symbol values are averaged when added up by adders 235 and236. It is through this noise reduction arrangement that the phasecorrection signals CS and SN are obtained. The delay time (average delaytime) T required for the averaging by the averaging circuit 43 is givenas

T=Ds×(N - 1)/2  

[0013] where N denotes the number of symbols used for the averagingoperation.

[0014] The data signals S_(I) and S_(Q) are both despreads by an inversedata spreading unit 42 (bottom left in FIG. 15) using the spread codesignal PN_(13 ID) for the signal I and the spread code signal PN_(—QD)for the signal Q. The operation causes four signals to be extracted. Thefour chip rate signals are converted by an accumulator 44 into symbolrates to become signals D₁ through D₄. After this, the signals D₁through D₄ are each delayed by a data delaying unit 48 (FIG. 17) by theaverage delay time T of the averaging circuit 43. The operation yieldssignals D₁₀ through D₄₀. Where the data delaying unit 48 is constitutedby a number of delay gates (Ds) in stages of cascade connection eachgate providing one-symbol period delay, the gate count M per stage isgiven as

M=(N - 1)/2  

[0015] In the above example, N=3 and thus M=1, so that the delay gates480 through 483 of the data delaying unit 48 are each composed of aone-symbol delay gate (Ds).

[0016] The signals D₁₀ through D₄₀ are fed to a phase correction circuit49 in which the signals are corrected in phase rotation by use of thecorrection signals CS and SN. A typical constitution of the phasecorrection circuit 49 is shown in FIG. 18. The phase correction circuit49 performs phase correction as follows: the signals D₁₀ and D₄₀ aremultiplied by the correction signal CS, and the signals D₂₀ and D₃₀ bythe correction signal SN. The multiplied results are added andsubtracted mutually so as to rotate the orthogonal axes of the receiveddata by -ø in phase (i.e., the phase shift ø is reduced to zero in FIG.14). The phase correction provides reconstructed signals I_(R) and Q_(R)of the original signals I and Q. The signals _(IR) and Q_(R) thenundergo QPSK demodulation, not shown, to become the original data.

[0017] One disadvantage of the conventional detection circuit above isthat the restored signals I_(R) and Q_(R) are unavoidably affected bythe frequency precision of the crystal oscillator 60 (right-hand side inFIG. 13). A transmitter 60 used in the mobile station necessarilyincludes a certain practical frequency error because the mobile stationis for use by general users. That is, on the one hand, if the frequencyerror involved in the data is large enough to cause apparent phaseirregularities over the average delay time T during data demodulation,no precise correction signals can be acquired and the bit error rate ofthe detected data worsens. On the other hand, if the average delay timeT is shortened to avert the deterioration of the bit error rate, theadverse effects of the frequency error are diminished but the line noisebecomes more pronounced.

[0018] On the up link (i.e., a link from the mobile station to the basestation), the carrier C_(m) from the crystal oscillator 60 often doublesas a carrier for use in radio frequency quadrature modulation by themodulation circuit of the mobile station. In that case, the signalstransmitted by the mobile station and received by the base stationinclude both the phase error caused by fading and the frequency errororiginating from the crystal oscillator. The frequency error results inthe inevitable deterioration of the bit error rate in the detectionprocess of the base station.

[0019] The deficiencies above are conventionally circumvented,particularly where data of lower bit rates than normal are transmitted,by the method of burst data transmission with no change in the spreadingratio, as stipulated by the U.S. digital radio communication standard IS(Interim Standard)-95. Under the system, transmitting data at 1/r of thestandard bit rate compresses the data to 1/r in temporal terms. Thetime-compressed data is transmitted in bursts at fixed intervals.

[0020] How the burst signals are sent intermittently is illustrated inFIG. 19. In FIG. 19, the axis of abscissa represents time and the axisof ordinate denotes transmission power. Reference numeral 140 is a radiosignal waveform of standard bit rate data. Reference numerals 141, 142and 143 stand respectively for radio signal waveforms of data at ½, ¼and⅛of the standard bit rate. The number of burst signals varies with thebit rate. All burst signals have the same standard bit rate whentemporally compressed as described. It follows that every burst signalhas the same symbol rate and thus the spreading ratio remains unchanged.

[0021] The arrangements above are necessitated by the following reasons:if compression is not carried out, the one-symbol period gets longer thelower the data rate. Meanwhile, the number of symbols N for use by theaveraging circuit 43 (FIG. 15) in the demodulation circuit remainssubstantially the same regardless of the bit rate in view of noisereduction. Thus the average delay time T becomes longer the lower thedata rate. A prolonged delay time T prompts the frequency error todeteriorate the bit error rate as discussed above. The lower the bitrate, the more deteriorated the bit error rate. To avoid this deficiencyrequires keeping the symbol rate constant. The requirement necessitatesthe use of complicated circuits in the mobile station, which runscounter to the inherent need for the mobile station to simplify itscircuitry.

SUMMARY OF THE INVENTION

[0022] It is therefore an object of the present invention to overcomethe above and other deficiencies and disadvantage of the prior art andto provide an improved CDMA mobile communication system permittingstable signal reception with a minimum of bit error.

[0023] In carrying out the invention and according to one aspectthereof, there is provided a CDMA mobile communication system includinga mobile station comprising a voltage-controlled oscillator and afrequency controller. The voltage-controlled oscillator acts as acircuit to supply a carrier to a radio frequency quadrature demodulator.The frequency controller detects a frequency error from a phasecorrection signal of the first step and uses the detected frequencyerror as the basis for generating a control signal for use by theoscillator. The frequency controller may illustratively be composed oftwo circuits: a circuit for detecting a phase change caused by thefrequency error derived from the phase correction signal of the firststep and from a signal preceding the correction signal by apredetermined delay time; and an integrating circuit for integrating thephase change and outputting the result as the above control signal.

[0024] The voltage-controlled oscillator and the frequency controlleroperate to establish within the detection circuit of the mobile stationa control loop whereby the phase change is reduced substantially tozero. This minimizes the frequency error. Because the frequency of theoscillator is kept as precise as that of the oscillator of the basestation, the phase shift attributable to the frequency error issignificantly reduced. This provides a detection circuit that worksstably with a minimum of bit error.

[0025] The predetermined delay time may preferably be set within a rangenot exceeding the delay time needed for the averaging operation by theaveraging circuit which admits the phase correction signal of the firststep and outputs a phase correction signal.

[0026] The carrier to a radio frequency quadrature modulator maypreferably be supplied by the voltage-controlled oscillator. Because thefrequency of the radio signal sent to the base station is kept accurate,the base station is allowed to implement pilot signal-based coherentdetection stably with a minimum of bit error. Where the mobile stationtransmits data at a low bit rate, the system allows the terminal to keepthe chip rate of the spread code constant and to transmit data withvarying spreading ratios but without time compression. Such datatransmission is readily implemented by changing the circuit constant inkeeping with the symbol rate of the data, with no change in the circuitconstitution.

[0027] These and other objects and many of the attendant advantages ofthe invention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a circuit diagram of a CDMA mobile communication systembased on a CDMA detection circuit and practiced as a first embodiment ofthe invention;

[0029]FIG. 2 is a schematic view showing a pilot signal as it is relatedto data in a signal transmitted by the base station of the firstembodiment;

[0030]FIG. 3 is a circuit diagram of the second half of the detectioncircuit in a mobile station of the first embodiment;

[0031]FIG. 4 is a circuit diagram of a frequency controller used by thefirst embodiment;

[0032]FIG. 5 is a circuit diagram of another frequency controller foruse by the first embodiment;

[0033]FIG. 6 is a schematic view depicting a pilot signal as it isrelated to data in a signal transmitted by the mobile station of thefirst embodiment;

[0034]FIG. 7 is a circuit diagram of the second half of the detectioncircuit in the base station of the first embodiment;

[0035]FIG. 8 is a schematic view illustrating pilot signals as they arerelated to data in a signal transmitted by the mobile station of thefirst embodiment;

[0036]FIG. 9 is a schematic view explaining data transmission by themobile station of the first embodiment;

[0037]FIG. 10 is a schematic view showing a pilot signal as it isrelated to data in a signal transmitted by a mobile station of a secondembodiment;

[0038]FIG. 11 is a circuit diagram of the second half of the detectioncircuit in the base station of a third embodiment;

[0039]FIG. 12 is a circuit diagram of a temporary judge circuit in thesecond half of the detection circuit shown in FIG. 11;

[0040]FIG. 13 is a circuit diagram of the modulation circuit in aconventional base station and the first half of the detection circuit ina conventional mobile station;

[0041]FIG. 14 is a schematic view of a receiving point as it is rotatedin phase;

[0042]FIG. 15 is a circuit diagram of the second half of the detectioncircuit in the conventional mobile station;

[0043]FIG. 16 is a circuit diagram of an averaging circuit in the secondhalf of the detection circuit;

[0044]FIG. 17 is a circuit diagram of a data delaying unit in the secondhalf of the detection circuit;

[0045]FIG. 18 is a circuit diagram of a phase correction circuit in thesecond half of the detection circuit; and

[0046]FIG. 19 is a schematic view depicting data transmission by theconventional mobile station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Preferred embodiments of the invention relating to a CDMA mobilecommunication system will now be described in detail with reference tothe accompanying drawings. In FIGS. 1 through 12, the component partswith their functionally identical or equivalent counterparts included inthe prior art examples of FIGS. 13 through 18 are designated by likereference numerals.

<First Embodiment>

[0048]FIG. 1 is a circuit diagram showing the overall constitution of aCDMA mobile communication system practiced as the first embodiment ofthe invention. The base station, which transmits data usually to aplurality of mobile stations, is shown sending data to a single mobilestation in FIG. 1. In the left-hand half of FIG. 1, reference numeral 1is a base station; 51 is a modulation circuit; 91 is a spread codegenerator; 54 is a radio frequency quadrature modulator; 61 is atemperature compensated crystal oscillator; 58 is a circulator forseparating a transmitted radio signal from a received radio signal; 12is the first half of a detection circuit; 62 is a radio frequencyquadrature demodulator; and 64 is a low-pass filter. Referencecharacters I and Q are an in-phase component signal and a quadraturesignal respectively; PN_(—ID) and PN_(—QD) are spread code signals forthe signals I and Q respectively; S_(I1) and S_(Q1) are an in-phasecomponent signal and an opposite-phase component signal, respectively,subject to the spreading of the output of the detection circuit firsthalf 12; C_(B) is a carrier output by the oscillator 61; and 55 is anair transmission channel.

[0049] In the right-hand half of FIG. 1, reference numeral 2 is a mobilestation; 52 is the first half of a detection circuit; 59 is a circulatorfor separating a received radio signal from a transmitted radio signal;57 is a radio frequency quadrature demodulator; 56 is a low-pass filter;and 63 is a voltage-controlled oscillator. Reference characters CM standfor a carrier output by the oscillator 63, and AFC for a control signalfor controlling the frequency of the oscillator 63. Reference numeral 70denotes a frequency controller for generating the control signal AFC.Reference characters CS_(s) and SN_(s) stand for phase correctionsignals of the first step, to be described later; and S_(I) and S_(Q)for an in-phase component signal and an opposite-phase component signalsubject to the spreading of the output of the detection circuit firsthalf 52. Reference numeral 11 is a demodulation circuit; 25 is a spreadcode generator; and 66 is a radio frequency quadrature modulator.Reference characters I₁ and Q₁ represent an in-phase component signaland a quadrature component signal, respectively, of the data transmittedby the mobile station to the base station; and PN_(—ID) and PN_(—QD)denote spread code signals for the signals I_(I) and Q_(l),respectively, output by the generator 25.

[0050] Described below is the case in which the base station 1 transmitsdata and a pilot signal over a down link to the mobile station 2. Themodulation circuit 51 is substantially the same in constitution as itsconventional counterpart in FIG. 13. The data to be transmitted passesthrough a QPSK modulator, not shown, to become signals I and Q. Thesignals I and Q are spread by use of the spread code signals PN_(—ID)and PN_(—QD). The spread signals are turned by the radio frequencyquadrature modulator 54 into mutually perpendicular radio frequency bandsignals that are transmitted from an antenna past the circulator 58. Thetemperature compensated crystal oscillator 61 supplies the modulator 54with the carrier C_(B.)

[0051] Although not shown, An in-phase signal I_(p) and a quadraturesignal Q_(p) of the pilot signals are spread respectively by spread codesignals PN_(—IP) and PN_(—QP). Both spread code signals have the samechip rate as in the case of data. The pilot signals thus spread aresubject to radio frequency quadrature modulation by the same carrierC_(B) as with data. Following the modulation, the signals turn intomutually perpendicular signals transmitted on the same radio frequencyband as with data.

[0052]FIG. 2 schematically shows a radio frequency band signaltransmitted by the base station 1. In FIG. 2, reference numeral 92 is aradio frequency band signal representing the pilot signal, and 93 is aradio frequency band signal that carries data. Data 2 in the signal 93is destined to the mobile station 52; data 1 and p are directed to othermobile stations. The data signals 1 and p are each spread by a differentspread code signal.

[0053] The data and pilot signals are thus transmitted on the same radiofrequency band and received by the mobile station 2 (right-hand half ofFIG. 1). The received signals are fed to the radio frequency quadraturedemodulator 57 past the circulator 59. The output of the demodulator 57,from which the low-pass filter 56 removes the spurious part, becomes thesignals S_(I) and S_(Q). The voltage-controlled oscillator 63 suppliesthe demodulator 57 with the carrier CM.

[0054] The signals S_(I) and S_(Q) are despread and phasecorrected bythe second half of the detection circuit. This yields restored signalsI_(R) and Q_(R) originating from the initial signals I and Q. FIG. 3 isa circuit diagram of the second half of the detection circuit in themobile station. The output terminals of the accumulator 41 are connectedto the input terminals of the frequency controller 70 which is fedthereby with the phase correction signals CS_(S) and SN_(S) of the firststep. Except for these connections, the setup of FIG. 3 is the same asthat of the conventional circuit in FIG. 15. The component parts havingtheir functionally identical or equivalent counterparts included in theprior art examples will not be described further.

[0055] The oscillator 63 (in the right-hand half of FIG. 1) is a knowncircuit using a variable capacitance diode (not shown) as the element todetermine the oscillation frequency. The diode has its capacitancechanged when fed with the control signal AFC, whereby the oscillationfrequency is controlled.

[0056] The frequency controller 70 that outputs the control signal AFCworks as follows: a phase shift of Δø is detected as a phase change ofabout one-symbol period stemming from the frequency error of theoscillator 63. The sine component (sinΔø) of the phase shift is fed toan integrator so that the latter will output the control signal AFC.FIG. 4 shows the circuit constitution of the frequency controller 70. InFIG. 4, reference numerals 700 and 701 are delay gates (Ds) having adelay time of one-symbol period each, 705 and 706 are multipliers, 707is a subtracter, 708 is a multiplier, and 709 is an integrator.

[0057] The signals CS_(s) and SN_(s) are delayed by the delay gates 700and 701. The multiplier 706 multiplies the signal SN_(s). by a signalsucceeding the signal CS_(s) by one symbol. The multiplier 705multiplies the signal CS_(s) by a signal succeeding the signal SN_(s) byone symbol. The subtracter 707 subtracts the product of the multiplier706 from that of the multiplier 705, yielding an error signal SNΔøhaving a value of sinΔø. If Δø<<φ, then sin Δø, is approximately equalto Δø. The error signal SNΔ having the value of sinΔø is multiplied bythe multiplier 708 to provide a predetermined loop gain. The multipliedresult is integrated by the integrator 709 that produces the controlsignal AFC.

[0058] The controller 70, oscillator 63 and radio frequency quadraturedemodulator 57 in FIG. 1 as well as the despreading unit 21 andaccumulator 41 in FIG. 3 constitute a control loop in which theintegrator 709 integrates the signal SNΔ so that the latter willapproach zero. This arrangement inhibits the frequency error and keepsthe frequency of the oscillator in the mobile station as accurate asthat of the oscillator in the base station.

[0059] The phase change Δø is also caused by the phase errorattributable to fading. However, the fading-triggered phase change isgenerally very slow and thus quite small compared with the change causedby frequency error. For a period of one symbol or thereabout, there ispractically no harm in assuming that the change Δø is caused solely byfrequency error.

[0060] The example explained above is one in which the processing of thecontroller 70 is carried out in a one-symbol period. If the frequencyerror is very small during the one-symbol period, it is possible toperform the processing of the controller 70 over a period involving aplurality of consecutive symbols. In this case, the period must notexceed the average delay time T for the averaging circuit 43 (FIG. 16).

[0061] Conversely, if the frequency error is relatively large during theone-symbol period, the processing needs to be carried out at a speedhigher than the symbol rate. FIG. 5 shows a circuit diagram of analternative frequency controller 70 performing its processing morequickly than the symbol rate. In FIG. 5, reference numerals 710 and 711are abstract code circuits, 712 and 713 are delay gates with their delaytime shorter than the one-symbol period, 714 and 715 are exclusive-ORgates, and 718 is an integral calculus. The abstract code circuits 710and 711 extract the signs (plus or minus) from the signals CS_(s). andSN_(s) respectively. The extracted signs indicate a quadratic movementof the pilot signal coordinates caused by the phase shift ø, as shown inFIG. 14. For example, if the phase shift ø falls within a range of 180through 270 degrees, the receiving point moves into the third quadrant,and the signals CS_(s) and SN_(s), have the minus signs. The abstractcode circuits 710 and 711 recognize the absence of frequency error (flag“O”) if the signals have the plus signs, or the presence of frequencyerror (flag “1”) if the signals have the minus signs. The flags “0” and“1” are output as sign signals “cos-flag” and “sin-flag” respectively.

[0062] The sign signal “cos-flag” and the sign signal “sin-flag” thathas passed the delay gate 713 are fed to the gate 714. The sign signal“sin-flag” and the sign signal “cos-flag” that has passed the delay gate712 are supplied to the gate 715. The output signals of the gates 714and 715 are sent to the integral calculus 718. If the gate 714 outputs“1”, then the integrator 718 outputs as the control signal AFC a voltagethat raises the frequency of the oscillator 63; if the gate 715 outputs“1”, the integrator 718 outputs as the control signal AFC a voltage thatlowers the reference frequency. Where the processing needs to beperformed faster than the symbol rate, as in this example, it ispossible to implement a high-speed frequency controller that dispenseswith multipliers carrying out time-consuming multiplications.

[0063] The voltage-controlled oscillator 63 and the two kinds offrequency controller 70 may each be constituted by a known semiconductorintegrated circuit. Thus constituted, the inventive setup isincorporated advantageously in mobile stations for use by general users.

[0064] Returning to FIG. 1, what follows is a description of the case inwhich the mobile station 2 transmits data and pilot signals over an uplink to the base station 1. The data to be transmitted undergoes QPSKmodulation (not shown) to become signals I_(I) and Q_(I) (bottom rightin FIG. 1). The signals I₁, and Q₁, are spread by the spread codesignals PN_(—ID) and PN_(—QD) from the spread code generator 25. Thesignals thus spread pass through the radio frequency quadraturemodulator 66 to become mutually perpendicular radio frequency bandsignals that are transmitted from an antenna past the circulator 59. Thevoltage-controlled oscillator 61 supplies the modulator 66 with thecarrier C_(M.)

[0065] In transmitting the pilot signal to the base station 1, themobile station 2 multiplexes the signal with the data on a time-divisionbasis. According to this method, the signals I₁, and Q₁, make up asignal form having the data and pilot signals multiplexed therein. Thedata and pilot signals are both spread by the spread code signalsPN_(13 ID) and PN_(—QD). FIG. 6 shows a radio frequency band signalmultiplexed in the manner described. In FIG. 6, reference numeral 94 isa pilot signal part, and 95 is a data part.

[0066] The signal received by the antenna of the base station 1 is sentto the radio frequency quadrature demodulator 62 past the circulator 58in the first half 51 of the detection circuit (bottom left in FIG. 1).The output signal of the demodulator 62, from which the low-pass filter64 removes the spurious part, turns into signals S_(I1) and S_(1Q). Thedemodulator 62 is supplied with the carrier C_(b). from the oscillator61. The signals S_(I1), and S_(Q1) are subject to despreading and phasecorrection in the second half of the detection circuit, to be describedlater. The despreading and phase correction processes provide thereconstructed signals I_(1R) and Q_(1R) originating from the initialsignals I₁, and Q_(1.)

[0067]FIG. 7 is a circuit diagram of the second half of the detectioncircuit in the base station 1. In FIG. 7, reference numeral 80 is areceived signal despreading unit; 91 is a spread code generator; 82 isan accumulator; 83 is a phase correction signal extracting unit thatextracts phase correction signals CS_(s1) and SN_(s1) of the first step;84 is an averaging circuit that receives the signals CS_(s1), andSN_(s1) from the extracting unit 83 and outputs phase correction signalsCS₁ and SN₁; 85 is a data extracting unit that extracts the data partfrom the signal converted to the symbol rate; 103 is a data delayingunit that delays the extracted data by the average delay time of theaveraging circuit 84: and 88 is a phase correction circuit that rotatesin phase the data from the delaying unit 103 and outputs the signalsI_(1R) and Q_(1R.)

[0068] The received signal despreading unit 80 despreads each of thereceived signals S_(I1) and S_(Q1) using the two spread code signalsPN_(—ID) and PN_(—QD) from the spread code generator 91. The four chiprate signals thus obtained are converted by the accumulator 82 intosymbol rate signals A₁ through A₄. The phase correction signalextracting unit 83 is supplied with the sum of the signals A₁ and A₄(including the cosine component of the pilot signal) on the one hand,and with the difference between the signals A₃ and A₂ (including thesine component of the pilot signal) on the other. The extracting unit 83extracts only the pilot signal part from the time-division multiplexedsignals so as to output the phase correction signals CS_(s1), andSN_(s1), of the first step. The averaging circuit 84 averages aplurality of symbols of the signals CS_(s1) and SN_(s1) to output thephase correction signals CS₁ and SN₁ for use in data phase rotation.

[0069] The signals A₁ through A₄ are also sent to the data extractingunit 85. The extracting unit 85 extracts only the data part from thetime-division multiplexed signals. The four-signal data thus obtained isforwarded to the data delaying unit 103. The delaying unit 103 delayseach of the received four signals and outputs data D₁₀₁, through D₄₀₁.The circuit constitution of the phase correction circuit 88 is the sameas that shown in FIG. 17.

[0070] With the first embodiment, the values of phase rotation by thecorrection signals CS₁, and SN₁, are set as indicated below. FIG. 8shows the received signal structured in units of symbols. In FIG. 8, apilot signal of h symbols and a data signal of j symbols are alternatelyreceived. Initially, the averaging circuit 84 averages the h symbols ofa pilot signal 98 and the h symbols of a pilot signal 100. The averagingoperation determines phase rotation quantities of øhl and øh2. Theamount of phase rotation per symbol of data 99 is given as

øh1(1-31 s/h)+-526 h2(s/h)  

[0071] where s stands for the s-th symbol (s=1−j). In this manner, thephase rotation is accomplished while the pilot signals preceding andsucceeding the data part are taken into consideration. This requiresdelaying the current data until the ensuing pilot signal is received.Thus the average delay time, i.e., the delay time of the delaying unit103, is determined as the j-symbol period of the data 99 supplemented bythe h-symbol period of the pilot signal 100.

[0072] Where the up link described above is in effect, the radiofrequency quadrature modulator 66 (bottom right in FIG. 1) of the mobilestation 2 is supplied with the carrier CM output and kept precise by thevoltage-controlled oscillator 63. This allows the base station 1 toavoid the problem of frequency error and to implement stable detection.That in turn makes it possible to adopt a spreading circuit that keepsthe chip rate of the spread code constant where the mobile stationtransmits data at a bit rate lower than the standard rate. If k isassumed to represent the spreading ratio in effect when the data bitrate is standard, the spreading ratio is changed to bk where the bitrate is 1/b (b≧1) of the standard bit rate.

[0073]FIG. 9 shows transmitted signals of different bit rates. In FIG.9, the axis of abscissa represents time and the axis of ordinate denotestransmission power. Reference numeral 160 is a signal that transmitsdata at the standard bit rate with a spreading ratio of k; 161 is asignal that transmits data at ½of the standard bit rate with a spreadingratio of 2k, powered by ½of the power level for the standard bit rate;162 is a signal that transmits data at ¼of the standard bit rate with aspreading ratio of 4k, powered by ¼of the standard power level; and 163is a signal that transmits data at ⅛of the standard bit rate with aspreading ratio of 8k, powered by ⅛of the standard power level. Intransmitting data at such different bit rates, the first embodimentimplements CDMA communication by varying the circuit constant in keepingwith the bit rate but without changes in the circuit constitution.

<Second Embodiment>

[0074] Described below is the second embodiment of the inventive CDMAmobile communication system in which a plurality of mobile stations areassigned different spread codes for their pilot signals, each mobilestation transmitting the pilot signal using the assigned spread codeover an up link to the base station. Data is transmitted by use of themodulation circuit 11 shown in the right-hand half of FIG. 1. Althoughnot shown, an in-phase signal and a quadrature signal of the pilotsignals are spread respectively by spread code signals having the samechip rate as in the case of data. The pilot signals thus spread aresubject to radio frequency quadrature modulation by the same carrierC_(B) as with data. Having undergone the modulation, the signals turninto mutually perpendicular signals transmitted on the same radiofrequency band as with data.

[0075]FIG. 10 schematically shows radio frequency band signalstransmitted by the mobile station 2. In FIG. 10, reference numeral 96 isa radio frequency band pilot signal, and 97 is a radio frequency banddata signal. The pilot signal is transmitted at a power level lower thanthe data signal. The transmitted signals are received by the basestation 1 constituted by the first half of the detection circuit 12 inthe bottom left portion of FIG. 1 and by a circuit having the sameconstruction as the second half of the detection circuit in FIG. 3.

[0076] The modulation circuit 11 in the mobile station 2 utilizes thecarrier C_(M) kept precise for radio frequency quadrature modulation.This allows the base station 1 to circumvent the problem of frequencyerror and to implement stable detection.

<Third Embodiment>

[0077] Described below is the third embodiment of the inventive CDMAmobile communication system which derives the phase correction signalsof the first step from the phase rotation changes of data, with no useof pilot signals for frequency control. With the third embodiment, thedata to be transmitted from the base station 1 is subject to BPSK(binary phase shift keying) modulation. Signals I_(b), and Q_(b). areacquired through the BPSK modulation. The modulation circuit of the basestation 1 and the first half of the detection circuit in the mobilestation 2 in connection with the signals I_(b), and QB are the same asthose shown in FIG. 1. The second half of the detection circuit in themobile station 2 is illustrated in FIG. 11. In FIG. 11, referencenumeral 45 represents a temporary judge circuit. Reference charactersCS_(CB) and SN_(CB) denote input signals to the temporary judge circuit45, and CS_(SB) and SN_(SB) indicate phase correction signals of thefirst step output by the temporary judge circuit 45.

[0078] The data despreading unit 42, spread code generator 25,accumulator 44, averaging circuit 43, data delaying unit 48, phasecorrection circuit 49 and frequency controller 70 in FIG. 11 are thesame in function as their counterparts of the first embodiment in FIGS.3 and 4. The signals CS_(SB) and SN_(SB) are supplied to the frequencycontroller 70 generating the control signal AFC for thevoltage-controlled oscillator 63 (FIG. 1). The signals CS_(SB) andSN_(SB) are also fed to the averaging circuit 43 that generates phasecorrection signal CS_(B) and SN_(B.)

[0079] In the second half of the detection circuit in the mobile station2 of FIG. 11, the signals S_(IB) and S_(QB) output by the detectioncircuit first half 52 (right-hand half in FIG. 1) are despread by thedata despreading unit 42 using the spread code signals PN_(—ID) andPN_(—QD) for the signals I_(B) and Q_(B) respectively. The despreadsignals are converted by the accumulator 44 from the chip rates tosymbol rate signals D_(IB) through D_(4B). The signals D_(IB) and D_(4B)are added up to yield the signal CS_(CB) representing the cosinecomponent of the data, and the signal D_(2B) is subtracted from thesignal D_(3B) to give the signal SN_(IB) representing the sine componentof the data. The signals CSCB and SNCB are fed to the temporary judgecircuit 45.

[0080] The data is composed of “1” and “O” iterations or of no changesper symbol (the pilot signal remains unchanged). Thus where the signalsCS_(CB) and SN_(CB) are both inverted in phase per symbol due to datachanges, it. is desired to generate signals that would correct the phaseinversion so as to render the input signals apparently unchanged with noshift in phase. Such signals, when generated by the temporary judgecircuit 45, serve as phase correction signals of the first stepfunctionally equivalent to those acquired by use of the pilot signal.

[0081]FIG. 12 is a circuit diagram of the temporary judge circuit 45. InFIG. 12, reference numerals 182, 183 and 189 are delay gates (Ds) havinga delay time of one-symbol period each; 184 and 185 are multipliers; 180is an adder; 186 is a abstract code circuit; 181 is an exclusive-ORgate; and 187 and 188 are sign inverting units.

[0082] The signal CS_(CB) is multiplied by a signal preceding the signalCS_(CB) by one symbol, and the product is fed to the adder 180. At thesame time, the signal SN_(CB) is multiplied by a signal preceding thesignal SN_(CB) by one symbol, and the product is supplied to the adder180. The result of the addition is sent to the abstract code circuit 186which outputs a signal indicating whether the signals CS_(CB) andSN_(CB) are simultaneously inverted in phase.

[0083] The output signal of the extracting unit 186 is sent to theexclusive-OR gate 181. The other input of the exclusive-OR gate 181 is asignal preceding by one symbol the output signal of the same gate. Theexclusive-OR gate 181 outputs “1” if the absence of the simultaneousphase inversion preceding a given symbol is replaced by the presence ofthe inversion following that symbol or vice versa; the exclusive-OR gate181 outputs “0” if the simultaneous phase inversion is either absent orpresent both before and after a symbol (if the simultaneous phaseinversion of the signals CS_(CB) and SN_(CB) continues before and aftera symbol, that means the original data is restored). With theexclusive-OR gate 181 outputting “1”, the sign inverting units 187 and188 output the input signals CS_(CB) and SN_(CB) after simultaneouslyinverting them in phase. Where the exclusive-OR gate 181 outputs “0”,the input signals CS_(CB) Band SNCB are output uninverted. The processabove turns the signals CS_(CB) and SN_(CB) into the phase correctionsignals CS_(SB) and SN_(SB) of the first step respectively.

[0084] The constitution and the workings of the frequency controller 70are the same as those of the first and the second embodiments. Given thephase correction signals CS_(SB) and SN_(SB) of the first step, thefrequency controller 70 outputs the control signal AFC to control thevoltage-controlled oscillator 63. With the third embodiment, the phaserotation of the data following despreading is corrected and thevoltage-controlled oscillator 63 is kept accurate as effectively as inthe case where the pilot signal is utilized. The third embodiment thuspermits the base station 1 and mobile station 2 to implement stabledetection. In particular, the mobile station 2 is allowed to realizedata transmission with an appropriate spreading ratio selected.

[0085] Although the first through the third embodiments adopt QPSK orBPSK modulation upstream of the spreading process, this is notlimitative of the invention. The invention is not dependent on thepre-spread modulation scheme because the invention aims to keep precisethe carrier for radio frequency modulation and demodulation. Any systemof pre-spreading modulation may be adopted in conjunction with theinvention. The invention, when suitably embodied, promises stableoperation in both coherent detection and differential detection.

[0086] According to the invention, the pilot signal acquired fromdespreading is used to detect frequency error, and the frequency of thecarrier is controlled so as to reduce the detected frequency error tozero. This allows the mobile station to implement stable detection witha minimum of bit error. Since the same carrier is used in radiofrequency quadrature modulation, the base station is allowed to realizestable detection with reduced bit error. When the mobile station is totransmit data at a low bit rate, an appropriate spreading ratio may beselected in accordance with the bit rate. This arrangement averts theprocess of keeping the spreading ratio constant——a process thatcomplicates circuitry. The features above make it possible to implementa more practical CDMA mobile communication system of higher performancethan ever before.

[0087] It is further understood by those skilled in the art that theforegoing description pertains to preferred embodiments of the disclosedsystem and that various changes and modifications may be made in theinvention without departing from the spirit and scope thereof.

What is claimed is:
 1. A code division multiple access mobilecommunication system for transmitting a pilot signal along with data andfor generating a phase correction signal from the despreading of thereceived pilot signal so that a phase error of the received data iscorrected by use of said phase correction signal during detection, thesystem comprising: a plurality of mobile stations and a base station;each of said plurality of mobile stations having a detection circuitincluding: a voltage-controlled oscillator for supplying a carrier to aradio frequency quadrature demodulator demodulating received radiofrequency band signals; and a frequency controller for detecting afrequency error from a first step phase correction signal fed to anaveraging circuit outputting said phase correction signal, saidfrequency controller further generating from said frequency error acontrol signal to control the frequency of said voltage-controlledoscillator; wherein said base station establishes up and down links inconjunction with said plurality of mobile stations.
 2. A code divisionmultiple access mobile communication system according to claim 1,wherein said frequency controller comprises: a phase change generatingcircuit for generating a phase change based on said frequency errorderived from said first step phase correction signal and from a signalpreceding said phase correction signal of the first step by apredetermined delay time; and an integrating circuit for integratingsaid phase change and outputting the integrated result as said controlsignal.
 3. A code division multiple access mobile communication systemaccording to claim 2, wherein said predetermined delay time is setwithin a range not exceeding the delay time needed for averaging by saidaveraging circuit.
 4. A code division multiple access mobilecommunication system according to claim 3, wherein said plurality ofmobile stations each include a radio frequency quadrature modulatorsupplied with the carrier from said voltage-controlled oscillator.
 5. Acode division multiple access mobile communication system according toclaim 4, wherein said plurality of mobile stations each set a spreadingratio variably in keeping with changing bit rates of data and generate aspread signal for transmission having a constant chip rate.
 6. A codedivision multiple access mobile communication system for transmittingand receiving data spread by a spread code over a radio frequency band,the system comprising: a plurality of mobile stations and a basestation; said plurality of mobile stations each having a detectioncircuit including: a temporary judge circuit acting when each of a signof a cosine component and a sign of a sine component of despread data isfound to be inverted one symbol later, said temporary judge circuitthereupon inverting the signs of the components individually andoutputting the sign-inverted components, said temporary judge circuitfurther outputting the components uninverted when the signs thereof arefound to be uninverted one symbol later; a frequency controller forgenerating a phase change based on the frequency error derived from theoutput signal of said temporary judge circuit and from a signalpreceding said output signal by a predetermined delay time, saidfrequency controller further integrating said phase change andoutputting the integrated result as a control signal; avoltage-controlled oscillator for receiving said control signal as asignal to control an oscillation frequency; and a radio frequencyquadrature demodulator for receiving the output signal of saidvoltage-controlled oscillator as a carrier to demodulate received radiofrequency band signals; wherein said base station establishes up anddown links in conjunction with said plurality of mobile stations.
 7. Amobile station for use with a code division multiple access mobilecommunication system for transmitting a pilot signal along with data andfor generating a phase correction signal from the despreading of thereceived pilot signal so that a phase error of the received data iscorrected by use of said phase correction signal during detection, saidmobile station having a detection circuit comprising: avoltage-controlled oscillator for supplying a carrier to a radiofrequency quadrature demodulator demodulating received radio frequencyband signals; and a frequency controller for detecting a frequency errorfrom a first step phase correction signal fed to an averaging circuitoutputting said phase correction signal, said frequency controllerfurther generating from said frequency error a control signal to controlthe frequency of said voltage-controlled oscillator.
 8. A mobile stationaccording to claim 7, wherein said frequency controller comprises: aphase change generating circuit for generating a phase change based onsaid frequency error derived from said phase correction signal of thefirst step and from a signal preceding said phase correction signal stepby a predetermined delay time; and an integrating circuit forintegrating said phase change and outputting the integrated result assaid control signal.
 9. A mobile station according to claim 8, whereinsaid predetermined delay time is set within a range not exceeding thedelay time needed for averaging by said averaging circuit.
 10. A mobilestation according to claim 9, further comprising a radio frequencyquadrature modulator supplied with the carrier from saidvoltage-controlled oscillator.
 11. A mobile station according to claim10, further comprising means for setting a spreading ratio variably inkeeping with changing bit rates of data and for generating a spreadsignal for transmission having a constant chip rate.
 12. A mobilestation for use with a code division multiple access mobilecommunication system for transmitting and receiving data spread by aspread code over a radio frequency band, said mobile station having adetection circuit comprising: a temporary judge circuit acting when eachof a sign of a cosine component and a sign of a sine component ofdespread data is found to be inverted one symbol later, said temporaryjudge circuit thereupon inverting the signs of the componentsindividually and outputting the sign-inverted components, said temporaryjudge circuit further outputting the components uninverted when thesigns thereof are found to be uninverted one symbol later; a frequencycontroller for generating a phase change based on the frequency errorderived from the output signal of said temporary judge circuit and froma signal preceding said output signal by a predetermined delay time,said frequency controller further integrating said phase change andoutputting the integrated result as a control signal; avoltage-controlled oscillator for receiving said control signal as asignal to control an oscillation frequency; and a radio frequencyquadrature demodulator for receiving the output signal of saidvoltage-controlled oscillator as a carrier to demodulate received radiofrequency band signals.
 13. A code division multiple access mobilecommunication system for transmitting a pilot signal along with data andfor generating a phase correction signal from the despreading of thereceived pilot signal so that a phase error of the received data iscorrected by use of said phase correction signal during detection, thesystem comprising: a plurality of mobile stations and a base station;each of said plurality of mobile stations having a detection circuitincluding: a voltage-controlled oscillator for supplying a carrier to aradio frequency quadrature demodulator demodulating received radiofrequency band signals; and means for detecting a frequency error from afirst step phase correction signal fed to an averaging circuitoutputting said phase correction signal, and for generating from saidfrequency error a control signal to control the frequency of saidvoltage-controlled oscillator; wherein said base station establishes upand down links in conjunction with said plurality of mobile stations.14. A code division multiple access mobile communication systemaccording to claim 13, wherein said frequency controller comprises:means for generating a phase change based on said frequency errorderived from said first step phase correction signal and from a signalpreceding said phase correction signal of the first step by apredetermined delay time; and means for integrating said phase changeand outputting the integrated result as said control signal.
 15. Amobile station for use with a code division multiple access mobilecommunication system for transmitting a pilot signal along with data andfor generating a phase correction signal from the despreading of thereceived pilot signal so that a phase error of the received data iscorrected by use of said phase correction signal during detection, saidmobile station having a detection circuit comprising: avoltage-controlled oscillator for supplying a carrier to a radiofrequency quadrature demodulator demodulating received radio frequencyband signals; and means for detecting a frequency error from a firststep phase correction signal fed to an averaging circuit outputting saidphase correction signal, and for generating from said frequency error acontrol signal to control the frequency of said voltage-controlledoscillator.
 16. A mobile station according to claim 15, wherein saidfrequency controller comprises: means for generating a phase changebased on said frequency error derived from said phase correction signalof the first step and from a signal preceding said phase correctionsignal step by a predetermined delay time; and means for integratingsaid phase change and outputting the integrated result as said controlsignal.